Caprolactam (i.e., 2-oxohexamethylenimine, hexahydro-2H-azepin-2-one) is one of the most widely used chemical intermediates; it a very important raw material for making man-made fibers, more specifically Nylon 6 polymers. Its annual consumption is in the range of millions of tons. Nylon 6 has been commercially used in a very wide variety of applications, such as: in the fiber industry for making Nylon fibers, cotton-like Nylon fibers; the fabric made from Nylon 6 can be used in making garments, panty hose, umbrella canopies, window curtains, carpets, etc; in fishing industry for making fishing nets, fishing lines, ropes, etc; in manufacturing industry for making tires, conveyer belts, transmission belts; for use as an engineering plastics for making gears, beatings, industrial parts, etc. Additionally, caprolactam itself can be used in many applications, such as making paints, plasticizers, synthetic leathers, lacquer, etc.
Conventionally the production of caprolactam uses benzene as the raw material, by which benzene is hydrogenated to form cyclohexane, which is then oxidized to become cyclohexanol and cyclohexanone. Subsequent reaction with ammonia-derived hydroxylamine forms cyclohexanone oxime which undergoes a Beckmann molecular rearrangement to from the seven-membered ring caprolactam, or more specifically, .epsilon.-caprolactam. The conventional process suffers several shortcomings, including relatively complicated manufacturing procedure and low reaction yield (only about 5 mole %). The conventional process also suffers from the problem of large reaction waste Typically, every kilogram of caprolactam production would generate about 2.8 kilograms of ammonium sulfate, a reaction by-product. These shortcomings cause the high cost of caprolactam to be maintained at an undesirably high level.
Alternative routes of producing caprolactam utilizing aminohexanoate (esters of aminohexanoic acids) have recently generated interests. German Pat. DE-2,249,993 discloses a process in which 6-aminohexanoate is heated, in a water-containing environment, at 250.degree..about.350.degree. C. to form caprolactam. The main disadvantage of this process is that the concentration of the raw material must be very low, so as prevent oligmers from being formed. The low caprolactam concentration in the reaction product, as a result of the low reactant concentration, causes the post-production separation very difficult and expensive.
German Pat. DE-3,235,938 discloses a process by which polyols with a boiling point greater than caprolactam, such as tetraethylene glycol, diglycerol, butanetriol, etc., are used as reaction solvent. With this process, thermal stability of the reactants can be a problem and large amounts of reaction by-products are produced, resulting in very poor selective yield of caprolactam. In Japanese Pat. 14,563, it is disclosed a process which uses ethanediol as reaction solvent at a reaction temperature of 160.degree..about.165.degree. C. for 4 hours. However, the results show that, not only the reaction yield of caprolactam was very low, it was very difficult to separate the reaction product caprolactam from the reaction solvent ethanediol. In U.S. Pat. No. 4,767,856, (the '856 patent) it is disclosed a process by which a heavy oil with a boiling point between 350.degree..about.550.degree. C., such as white oil, vacuum oil, molten wax, etc., was used as reaction solvent. Results from '856 process showed good conversion from methyl aminohexanoate and good selectivity of caprolactam. However, the reaction must be conducted at temperatures of 250.degree..about.330.degree. C., and the raw material concentration was limited to 3.about.7.5 wt %. These are the some of disadvantages of the '856 process.
German Patent DE-3,843,793 and U.S. Pat. No. 4,963,673 discloses a process in which aromatics having a boiling point between 110.degree..about.200.degree. C., such as toluene, xylene, etc, were used as the reaction medium, with a small amount of water added to increase the solubility of the raw material. This reaction medium ameliorates the separation problem that exists between caprolactam and solvent discussed above. Two types of reactors were disclosed: a continuous fixed bed type reactor and a batch type high pressure reactor. Reaction results indicated that with a fixed bed reactor, when the concentration of methyl aminohexanoate exceeded 9.3 wt %, the yield of caprolactam would decrease to below 65 mol %. Additionally, the use of toluene or xylene often caused phase separation problems in the intermediate reaction products, and substantial amounts of water must be added while, at the same time, the concentrations of toluene or xylene must be increased to at least 80 wt % in order to increase the solubility of the raw material (i.e., reactants). This would result in the expenditure of large amounts of effort for subsequent solvent recovery and causing problems during mass productions. On the other hand, with a batch type reactor, the reaction yields of caprolactam were all less than 80 mol %.
German Patent DE-3,823,213 discloses a process by which the reaction yield of caprolactam can be increased to 82 mol %. However, this process requires a reaction temperature of 330.degree. C., and generates large amounts of reaction by-products.
In summary, in order to achieve economically acceptable reaction yield of caprolactam, most of the prior art processes were conducted at reaction temperatures of 270.degree. C. or higher, and the reaction pressure typically exceeds 70 bar. Furthermore, in order to avoid the production of side-products (including polymers and/or oligmers), the reactant concentration in the feed was typically limited to less than 10 wt %. Catalysts were avoided in caprolactam manufacturing, mainly because of their tendency to cause undesired polymerization or oligmerization, which would lower the reaction yield and cause post-reaction separation problems. Furthermore, the high reaction temperature of the caprolactam manufacturing processes also tend to cause catalysts to be quickly deactivated, thus rendering a catalyzed process economically unattractive.